1. Field of the Invention
The present invention relates to switching power supplies, and more particularly, to a dynamic power factor correction control circuit for a switching power supply that regulates the current wave shape to match the input voltage wave shape in order to obtain close to unity power factor.
2. Description of Related Art
In view of the ever-increasing number of electronic devices that require a direct current (DC) source voltage, power supply circuits are well known in the art for converting an alternating current (AC) line voltage into a DC voltage. Such power supply circuits are known to include a full wave rectifier that converts the AC line voltage to a haversine signal, and a switching converter that converts the haversine signal to a relatively high DC output voltage level (e.g., 360 volts). The DC output voltage may be further reduced to a lower DC voltage level usable by an electronic device by coupling the power supply circuit to additional DC-to-DC converter circuits. Power supply circuits of this type tend to have a poor effective power factor (i.e., the ratio of true power to apparent power in an AC circuit) since they draw input current in short pulses of high peak value such that the current waveform is not sinusoidal. In order to maximize the actual power that can be drawn from a power supply, it is known to include a power factor correction (PFC) control circuit that controls the magnitude and phase of the input current to be sinusoidal and match the line voltage.
One conventional type of PFC control circuit derives a reference signal for the input current from a multiplier that scales the reference signal according to the deviation of the output voltage from its desired value. The output voltage is sampled by a voltage divider stage to provide an output reference signal that is feed to an error amplifier. The output of the error amplifier is then sampled by a sample/hold stage to scale the multiplication process. A drawback of this type of PFC control circuit is that the output power is dependent upon the square of the input voltage. This is undesirable since the purpose of the PFC control circuit is to either make the load appear to the line as a resistor (i.e., current proportional and in phase to voltage) or to supply power to a load that is relatively constant and independent of line fluctuations while improving its power factor. To address this drawback, it is also known to include a feed forward loop that divides the output of the error amplifier by the square of the input voltage. These circuits have limitations associated with the use of the arithmetic devices (i.e., multipliers and dividers), such as scaling errors, offsets and drifts, as well as increased complexity and associated cost due to the multiplicity of circuit components.
Accordingly, it would be very desirable to provide a simplified power factor correction control circuit for a switching power supply that regulates the current waveshape to match the input voltage waveshape.